Every impression is for some time retained in its sensible object, and that impression is longer retained in its object which was of greater power; and thus less so by the less powerful. - In this case I call 'sensible' that object which by some impression is moved from what it was before; an 'insensible' object is one that, though moved from its first being, retains in itself no impression of the thing that moved it. The sensible impression is that of the blow received in a sonorous thing, such as bells and similar things, and like the voice in the ear, which, if it did not retain the impression of the voices, a single song would never have grace; for in leaping from the first note to the fifth it is as though one heard those two notes at one time, and one hears the true consonance the first makes with the fifth; and if the impression of the first did not remain in the ear for some space of time, the fifth that follows immediately after the first would seem alone, and a single voice makes no consonance, and so any song made of a single note would seem to be without grace. Likewise the splendour of the sun or other luminous thing remains a while in the eye after it is seen; and in motion a single brand of fire, whirled swiftly in a circle, makes that circle appear of continuous and even glowing; the little grains of falling rain appear as continuous threads descending from their clouds; and thus it is shown that the eye retains the impressions of the moving things it sees. The insensible objects that do not retain the impressions of the things before them are mirrors and any lustrous thing which, the moment the thing impressed in it is taken from before it, at once remains wholly deprived of that impression. We shall therefore conclude that 'the motion of the mover impressed in the body moved by it is what moves that body through the medium wherein it moves.' Again, among the impressions retained in bodies may be placed the wave and the eddies of waters, and the winds in the air, and the knife stuck upon the table which, bent to one side and then left free, keeps a long trembling motion; which are all motions reflected one from another and may all be called incident up to the perpendicular of the place where that knife is fixed by its point. The voice impresses itself through the air without any flooding of air and strikes the objects and returns back to its cause. The percussion of liquid bodies with dense ones is of another nature than the percussions aforesaid. And the percussions of liquids with liquids again differ from the preceding percussions. Of the percussion of dense bodies with liquids, experience has been seen on the sea shores, which receive the waters against their rocks and drive them among the steep beaches; and it often happens that the course of the wave is not yet half spent when the stones it has carried return to the sea whence they departed. Their ruin is increased by the power of the wave that falls back from the high beaches and are re...